This relates generally to image sensors, and more specifically high efficiency image sensor pixels with deep trench isolation structures and embedded reflectors.
Modern electronic devices such a cellular telephones, cameras, and computers often use digital image sensors. Imagers (i.e., image sensors) include a two-dimensional array of image sensing pixels. Each pixel includes a photosensor such as a photodiode that receives incident photons (light) and converts the photons into electrical charges. Conventional image pixel arrays include frontside illuminated image pixels or backside illuminated image pixels. Image pixels are fabricated on a semiconductor substrate using complementary metal-oxide-semiconductor (CMOS) technology or charge-coupled device (CCD) technology. The image sensors include photodiodes and other operational circuitry such as transistors formed in a front surface of the substrate. In a configuration that includes frontside illuminated image pixels, a dielectric stack is formed on the front surface of the substrate directly on top of the photodiodes. The dielectric stack includes metal routing lines and metal vias formed in dielectric material. Image light passes through the dielectric stack to the photodiodes. In a configuration that includes backside illuminated image pixels, a thin dielectric is formed on the back surface of the substrate directly on top of the photodiodes. The thin dielectric stack includes High-K dielectric materials and anti-reflective materials. Image light passes through the thin dielectric stack to the photodiodes.
However, in conventional frontside and backside illuminated image pixels, light that is not absorbed as it passes through the photodiode does not generate charge in the region of the photodiode and instead passes deeper into the substrate or is leaked into other pixels where the charge it generates may be undesirable. This undetected light can negatively affect the quantum efficiency and color accuracy of the pixel and can limit the performance of the device, particularly in low-light imaging operations. Infrared and near-infrared wavelengths of light are particularly susceptible to this phenomenon, as conventional photodiodes are less efficient at converting these wavelengths of light into electrical charge. Additionally, some of the light can be scattered or diffracted into neighboring pixels and/or photon-generated charge diffuse into neighboring pixels thus creating pixel crosstalk and further degrading pixel performance and image contrast. Isolation structures in traditional image pixels can only extend to a limited depth before compromising spatial accurate imaging (i.e., the width of isolation structures causes the isolation structures to have too large of a footprint, which reduces the maximum achievable pixel density).
It would therefore be desirable to simultaneously maximize quantum efficiency and minimize electrical and optical crosstalk between neighboring pixels in an array of image sensor pixels.